1. Field of the Invention
The present invention relates to a method of forming a metallic member, a joining metallic member, and a metallic member joint.
2. Description of the Related Art
In a joint using an aluminum alloy tube, a cross-joining type of tubular metallic member joint is generally used, in which tubular metallic members are crossed each other so that an end of one of the tubular metallic member is joined to a surface of the other tubular metallic member. However, such a cross-joining type of aluminum alloy tube joint has a difficult in securing, particularly, bonding (joining) strength and joint strength as the diameter of each of the tubes or tubular members increases.
If a flange having a saddle-like curved surface or the like can be freely formed at the tube end of the aluminum alloy tube to conform with the outer surface shape of another tube material to be bonded at the aluminum alloy tube joint, bonding is facilitated, and joint strength is easily secured.
It has been proposed to apply an electromagnetic forming technique for forming such a flange. The electromagnetic forming technique is a technique for plastically forming or shaping of a workpiece into a predetermined shape by a method in which electric energy (electric charge) stored at a high voltage is input (discharged) into a current-carrying coil in a moment to form a strong magnetic field for a very short time, and thus a work (a work piece or metallic member) placed in the magnetic field is plastically deformed at a high speed by a strong expansive force or contractive force due to the repulsive force (Lorentz force according to the Fleming's left-hand rule) of the magnetic field.
For a metallic member such as a metal plate or tube having high conductivity and easily causing an eddy current, the electromagnetic forming technique has been conventionally considered promising for forming a plate, expanding a tube, contracting a tube, joining tubes, forming a tube end, etc. Particularly, an aluminum alloy is a good electric conductor, and is thus considered as a material suitable for the electromagnetic forming.
However, an electromagnetic method of forming the above-described flange at an end of an aluminum alloy tube has not yet been put into practical use. This is because the coil used for the electromagnetic forming has a short life due to a delay of development in an apparatus. However, particularly, tube expansion such as expansion of an end of an aluminum alloy tube has a problem of large difficulty in forming, as compared with tube contraction in which the diameter of a tube used for caulking is decreased.
Particularly, the above-described aluminum alloy tube joint is required to have high dimensional accuracy and shape accuracy over the entirety of the joint. Free tube expansion without using a mold such as a metal mold or the like cannot be put into practical use because an expanded portion formed by the electromagnetic forming method has low dimensional accuracy. Namely, in conventional electromagnetic forming of an end of an aluminum alloy tube by free tube expansion, a defect of the shape easily occurs as the diameter of the aluminum alloy tube increases and the size of the flange to be formed increases. Therefore, the flange having satisfactory dimensional accuracy and shape accuracy cannot be formed.
Therefore, in Mechanical Engineering Laboratory Report No. 150 “Research of Plastic Forming Using Electromagnetic Force” (March, 1990, issued by Mechanical Engineering Laboratory), it is proposed that tube expansion using a mold such as a metal mold or the like is required for integrally forming a flange with satisfactory dimensional accuracy and shape accuracy at an end of an aluminum alloy tube by electromagnetic forming. The electromagnetic forming method of Mechanical Engineering Laboratory Report No. 150 will be described in detail below with reference to FIG. 1.
However, even when the end of the aluminum alloy tube is expanded by using the mold, as described in Mechanical Engineering Laboratory Report No. 150, the formed flange has not very high dimensional accuracy and shape accuracy even by using a relatively thin plate of about 1 mm in thickness or a relatively narrow aluminum alloy tube having an inner diameter of less than 50 mmΦ.
Also, in the tube expansion of the end of the aluminum alloy tube by using the mold, the end of the aluminum alloy tube collides with the mold by expansion, thereby causing the problem of inevitably decreasing the thickness of the formed flange. This phenomenon tends to increase as the diameter of the aluminum alloy tube and the size of the flange formed increase. When the thickness of the formed flange decreases, joint strength deteriorates at the aluminum alloy tube joint in which the edge of the flange is joined by welding or a mechanical means. In welding, a thermal effect increases the deterioration in the joint strength.
Furthermore, in order to satisfy the dimensional accuracy and shape accuracy of the formed flange and suppress a reduction in thickness, another means can also be considered, in which the mold such as a metal mold or the like is used, and electromagnetic forming is performed stepwisely by a plurality of times of discharge of a current-carrying coil, not one time of discharge. However, in this case, the aluminum alloy is softened by the heat generated by repeated uses of the current-carrying coil to cause the problem of decreasing the strength. Also, a plurality of times of discharge of the current-carrying coil is expensive and deteriorates the process efficiency, and thus this method is not said to be practical. Therefore, in the actual situation, an electromagnetic method of forming the above-described flange at the end of the aluminum alloy tube has not yet been put into practical use.